1. Field of the Invention
The present invention relates to an optical low-noise downconverter, multiple dwelling unit, and related satellite television system, and more particularly, to an optical low-noise downconverter, multiple dwelling unit, and related satellite television system of low cost.
2. Description of the Prior Art
Satellite communication has several advantages, such as a wide coverage, low terrestrial interference, etc. For a satellite television (TV) broadcast system, the satellite TV broadcast system can provide users with various TV channels of high definition quality. Please refer to FIG. 1, which is a schematic diagram of a conventional satellite TV system 10. The satellite TV system 10 is a satellite master antenna television system suitable to a building or community, and each user in the building or community can receive satellite signals, such as 10.7-12.75 GHz Ku band satellite signals, through a master antenna. In the satellite TV system 10, the satellite signals are received by a satellite receiver 100, down-converted into an intermediate-frequency (IF) signal with a frequency band range of 0.95-2.15 GHz, and transmitted to a plurality of decoding devices 104 (such as set top boxes) of end users via a multiswitch 102, so as to be decoded. Under such a situation, all of the end users in the building or community can watch various satellite TV programs via a display device 106, such as a TV set. In addition, a terrestrial TV broadcasting signal is received by an antenna 108 and transmitted to the end-users via the multiswitch 102.
The satellite receiver 100 includes a dish reflector 110 and a low-noise block downconverter with feedhorn (LNBF) 112. The LNBF 112 includes a feedhorn antenna, an orthomode transducer (OMT), and a low-noise block down-converter (LNB). The feedhorn antenna is utilized for receiving a satellite signal reflected by the dish reflector 110, and the OMT is utilized for separating the satellite signal into a vertical and a horizontal polarization signals, outputted to the LNB. The LNB down-converts the vertical and horizontal polarization signals into four IF signals with different frequency bands, including a vertical low polarization signal VL, a vertical high polarization signal VH, a horizontal low polarization signal HL, and a horizontal high polarization signal HH, outputted to the multiswitch 102. A signal is transmitted from the satellite receiver 100 to the decoding device 104 of the end-user via a coaxial cable. In recent years, optic fiber cables are substituted for coaxial cables in parts of satellite TV systems, for significantly reducing transmission loss and satisfying the public with satellite TV programs of high definition quality.
Please refer to FIG. 2, which is a schematic diagram of a conventional satellite TV system 20. The satellite TV system 20 can simultaneously receive satellite signals from a plurality of satellites and transmit the satellite signals via optical fibers. Each of satellite receivers 200_1-200_N respectively receives a satellite signal transmitted from different satellites, down-converts the satellite signal into an IF signal, converts the IF signal into an optical signal, and transmits the optical signal to a headend equipment 202 for signal processing, such as demodulation or mixing. A signal outputted from the headend equipment 202 is transmitted to a multiple dwelling unit (MDU) 206 via a splitter 204. The signal is transmitted from the headend equipment 202 to the MDU 206 via an optical fiber cable. The MDU 206 includes tens of output ports and is set in a building or community with high density end-users. The MDU 206 is utilized for converting an optical signal into an electric signal as well as down-converting the electric signal for generating a signal being received by a decoding device 208 of the end-user. For conveniently illustrating, only the splitter 204, the MDU 206, and the decoding device 208 are notated in FIG. 2. In practice, as shown in FIG. 2, the headend equipment 202 is connected to a plurality of splitters, each splitter is connected to a plurality of MDUs, and each MDU is connected to a plurality of decoding devices of end-users.
In the satellite TV system 20, the LNBF uses an optical LNB for meeting the characteristic of optical fiber transmission, and the MDU 206 is designed to conform with the optical fiber transmission as well. Please refer to FIG. 3, which is a schematic diagram of a conventional optical LNB 30 that conforms to the universal LNB specification. In the optical LNB 30, a vertical polarization signal SV is transmitted and processed through a low-noise amplifier 300, a band-pass filter 302 with a passband range of 10.7-12.75 GHz, a mixer 304 for mixing the transmitted vertical polarization signal SV with a 9.75 GHz oscillating signal generated by an oscillator 306, a band-pass filter 308 with a passband range of 0.95-3.0 GHz, and an IF amplifier 310, for outputting an IF signal IF1. A horizontal polarization signal SH is transmitted and processed through a low-noise amplifier 312, a band-pass filter 314 with a passband range of 10.7-12.75 GHz, a mixer 316 for mixing the transmitted horizontal polarization signal SH with a 7.3 GHz oscillating signal generated by an oscillator 318, a band-pass filter 320 with a passband range of 3.4-5.45 GHz, and an IF amplifier 322, for outputting an IF signal IF2. The IF signals IF1 and IF2 are converted into optical signals by an optical transmitter 324 and transmitted via an optical fiber. From the above, the optical LNB 30 spreads the spectrum of the vertical and horizontal polarization signals inputted to the optical LNB 30 and transmit the vertical and horizontal polarization signals in different frequency bands.
Please refer to FIG. 4, which is a schematic diagram of a conventional MDU 40. The MDU 40 can be the MDU 206 in the satellite TV system 20. In the MDU 40, an optical signal inputted via an optical fiber cable is converted into an electric signal by an optical receiver 400 and is outputted to an IF amplifier 402. A signal outputted from the IF amplifier 402 is respectively filtered by band-pass filters 406 and 408 to generate IF signals IF3 and IF4 with different frequency bands. The IF signal IF3 is amplified, mixed, and filtered by two different circuits respectively and converted into a vertical low polarization signal VL and a vertical high polarization signal VH. Similarly, the IF signal IF4 is also amplified, mixed, and filtered by two different circuits respectively and converted into a horizontal low polarization signal HL and a horizontal high polarization signal HH. These horizontal and vertical polarization signals VL, VH, HL, and HH are outputted to decoding devices of end-users via a multiswitch 410.
Passband ranges of filters notated in FIG. 4 are corresponding to the optical LNB 30 of FIG. 3. For performing down-converting, the MDU 40 has to use oscillators 412 and 414 to generate a 0.85 GHz oscillating signal and a 3.3 GHz oscillating signal and a mixer 416 performing signal mixing for generating a 2.45 GHz oscillating signal. In addition, the MDU 40 has to use filters 418, 420, and 422 to prevent the oscillating signals from interfering with each other. In FIG. 4, in order to implement optical fiber transmission, there are a lot of components included in the MDU 40.
Since the optical transmitter 324 in the optical LNB 30 has to convert a 5.45 GHz electric signal, which is of a high frequency, cost of a laser diode in the optical transmitter 324 is three to four times of that of a common laser diode. Also, cost of a photodetector in the optical receiver 400 of the MDU 40 is several times of that of a common photodetector. In addition, since the IF amplifier 322 in the optical LNB 30 must be able to amplify an IF signal with a frequency band range of 3.4-5.45 GHz, cost of the IF amplifier 322 is several times of that of a common IF amplifier. In summary, the satellite TV system 20 using optical fiber cables is more expensive than the traditional satellite TV system using coaxial cables.